Metals can be explosively welded together, a process discovered through the use of artillery. The way that this was discovered was that various types of driving bands and seals have been used over the years to improve the fit of artillery shells in the bore of the gun. Sometimes this was in the form of a plate. Copper plates were used because they are soft metal and thus preserve the bore of the gun. However, it was discovered that the plates became permanently fused to the projectile after firing. The process is capable of bonding metals together that are metallurgically incompatible. Metal blanks for United States sandwich coins are made this way.